Experiments proposed in this application are designed to examine the mechanisms by which sound direction affects information processing in the frequency domain. We have previously shown that sound direction can significantly alter frequency threshold characteristics (FTCs) of neurons in the frog torus semicircularis (TS), a homolog of the mammalian inferior colliculus. Primarily, the FTC of most toral neurons becomes more sharply tuned when the free-field loudspeaker is rotated from the contralateral side to the ipsilateral side, or to the front of the animal. The mechanisms underlying these direction-dependent changes in the FTC of midbrain neurons are unclear and will be investigated in the next granting period. We hypothesize that changes in tuning width with sound direction are likely a consequence of binaural processing of the changing balance of inputs from the two cars. Single-unit recordings will be made from the frog TS to address four specific aims. Aim #1 is to determine whether or not a unit's binaural interaction pattern (EE, EI, or EO) gives rise to a specific direction-dependent FTC change. Aim #2 is to investigate the effects of monaural occlusion (e.g., the ipsilateral ear) on FTCs derived from free-field stimulation at different sound directions. The goal here is to assess the degree to which binaural interaction contributes to the direction-dependent sharpening of a unit's FTC in TS neurons displaying the different binaural interaction patterns. Aim #3 is to investigate whether or not the direction-dependent frequency selectivities of a unit can be mimicked by independently-controlled stimulation of the two ears. The relative contributions of interaural time (ITD) and level (ILD) differences for the sharpening of frequency tuning will be determined. Aim #4 is to investigate in E1 total neurons the FTCs at 2-3 azimuths under three different conditions: (a) normal intact, (b) when the recording locus receives a micro injection of antagonists of GABA-a or GABA-b receptors, (c) when the ipsilateral ear is occluded. The goals here are: (i) to determine whether or not binaural inhibition in the frog TS involves GABA, and GABA-based binaural inhibition is involved in direction-dependent sharpening of the FTC, (ii) to assess where the primary locus of binaural inhibition may be, i.e., whether or not it occurs mainly at the level of TS or preceding binaural convergent loci in lower brainstem. Results deriving from this study will shed light on the mechanisms by which binaural processing improves frequency discrimination.